In recent years, in order to improve fuel efficiency of an automobile, reduction in drive loss of an automatic transmission such as an AT or a CVT has been required. A seal ring is mounted to the automatic transmission for the purpose of hydraulic sealing. However, a friction loss of the seal ring leads to the drive loss of the automatic transmission. Therefore, reduction in friction of the seal ring is an important issue. Further, a capacity of an oil pump for the automatic transmission greatly affects the drive loss. Therefore, reduction in amount of leakage of oil through a gap between the seal ring and a shaft annular groove or a housing and reduction in capacity of the oil pump have been desired. In order to reduce the drive loss of the automatic transmission so as to improve the fuel efficiency of the automobile, low friction performance and high oil sealing performance have been demanded for the seal ring.
FIG. 1 is an illustration of a basic structure of a hydraulic sealing device to which the seal ring is mounted. Seal rings 1 are mounted to shaft annular grooves 4, which are formed in an outer peripheral surface of a shaft 2 so as to be located on both sides in an axial direction of a hydraulic oil passage 3. The hydraulic oil supplied from the hydraulic oil passage 3 is received with a seal ring pressure-receiving surface 11 and an inner peripheral surface 12. With this, a seal ring outer peripheral surface 13 is brought into contact with an inner peripheral surface of a housing 5, and a seal ring contact surface 14 is brought into contact with a side surface of the shaft annular groove 4, thereby preventing leakage of the hydraulic oil. In general, the shaft 2 rotates, and the housing 5 is fixed. However, an interchanged combination is possible.
In order to reduce the friction (friction loss) of the seal ring, there has typically been employed a technique of reducing a received pressure load generated by pressing the seal ring contact surface serving as a main sliding surface against the shaft annular groove. Specifically, a seal ring having a sectional shape formed so as to receive a hydraulic pressure of the hydraulic oil between the seal-ring contact surface and the shaft annular groove is adopted to reduce the received pressure load on the shaft annular groove, which is caused by the hydraulic pressure of the hydraulic oil received on the seal ring pressure-receiving surface.
In Patent Literature 1, there is disclosed a resin seal ring. The resin seal is connected in a ring-like shape by connecting abutment joint portions each having a composite step cut shape, and a thinned portion having a polygonal or convex sectional shape is formed on an inner diameter surface side of both side surface portions of the seal ring so as to continue over an entire periphery of the seal ring.
In Patent Literature 2, there is disclosed a seal ring. The seal ring is mounted to an annular groove formed in one of members corresponding to a housing having a shaft hole and a rotary shaft to be inserted into the shaft hole, and is brought into contact with a surface of another member. And it is brought into slidably contact with a side wall surface on a non-sealed fluid side of the annular groove so as to seal an annular gap between the members. A V-shaped concave portion along a ring circumferential direction, which serves as a non-contact portion with the side wall surface, is formed at least on a part of an inner-diameter side end portion of a ring side surface which serves as a sliding surface sliding with the side wall surface.